Digital image processing has become a significant form of image processing because of continuing improvements in techniques and increasingly powerful hardware devices. Digital image processing techniques have augmented and, in some instances, replaced methods used by photographers in image composition and dark room processing. For example, digital image processing techniques such as contrast balancing, edge sharpening, color balancing, or retouching of distortions are employed for editing original photographic images. Moreover, with the aid of a computer, digitized images can be edited to achieve a variety of effects such as changing the shapes and colors of objects and forming composite images.
In particular, digital image processing techniques are utilized to remove unwanted objects from original print images such as photographs. These unwanted objects can be distortions of the image resulting from the deterioration of the print image caused by normal wear or catastrophic events such as water damage. The unwanted objects can also be naturally occurring in the image. For example, facial blemishes or freckles may be considered unwanted spots in the digital image.
There are several types of unwanted objects that most often appear in digital images. The types are characterized by their shapes. One type is elongated and often results from folds or scratches in original image prints. This type of unwanted object can be automatically removed using a technique described in our co-pending U.S. patent application Ser. No. 08/964,629 entitled "AUTOMATED REMOVAL OF NARROW, ELONGATED DISTORTIONS FROM A DIGITAL IMAGE" to Lawton and Qadir, filed on Nov. 5, 1997. Another type appears as spots in the displayed digital image.
Spots in the digital image are caused by many things. For example, spots in an original print from which a digital image is derived may be the effects of the print's aging. Sudden distortion of the image can result from either mechanical or chemical sources. For example, the print can be scraped or folded, which may distort the image. Exposure of the print to water can cause distortion in the image. The imaging process may be subject to electrical noise that manifests itself as spots in the digital image that has a dust or snow-like appearance. Improper film development is another possible source of distortion that may result in spots in the original print and, in turn, in the digital image. The scanning of print images sometimes picks up dust from the scanning surface, which appears as spots in the digital image. Unwanted spots can also be naturally occurring objects in the image. For example, facial blemishes or freckles may be considered unwanted spots.
As used herein, the terms "spot" and "spots" refer to a wide range of visual effect, some of which are identified above. Each of these spots in digital images is characterized by a compact and generally homogenous appearance that is distinctly different than the surrounding portions of the image.
Using known digital image processing techniques, it is possible to automatically remove some types of spots from digital images. When the spots are small and appear as dust or snow in the digital image, a known technique for their automatic removal employs a median filter that attenuates the brightness of the pixels in the image having the most extreme brightness values. Unfortunately, this technique does not distinguish between a pixel whose brightness is a characteristic of an unwanted spot and a pixel whose brightness is a natural and desired attribute of the image. As a result of this inability to distinguish between these two types of pixels, all pixels are effected, which tends to degrade the quality of the image.
For example, one current imaging application sold under the brand name "ADOBE PHOTOSHOP" offers a correction tool which applies a median filter over the complete image for the purpose of removing noise through the blurring thereof to the point where it is no longer noticeable. Unfortunately, this process also tends to blur the details of the remaining portions of the digitized image causing a degradation in overall image quality. Another imaging application, sold by Metatools Inc. under the brand name Kai's Photo Soap includes an interactive user interface that assists the user to paint over a spot.
The foregoing approach for enhancing the quality of a digital image is just one example of several known ways to use pixel averaging techniques for automatically removing noise from a digital image that appears as snow or dust. Other known approaches for enhancing image quality include, for example, Fourier Frequency Transform filtering techniques for automatically removing noise that appears in a spatially regular pattern throughout the image.
Large spots in the digital image, which appear more like blotches than snow or dust, are not amenable to correction using any of the foregoing techniques. Instead, common techniques for removing such blotchy spots employ digital painting techniques. These techniques are not automatic and they depend on individual artistic and computer skills to subjectively cover or replace the area of the image occupied by the spot with a corrected image in a manner that simulates painting a canvas. Thus, the quality of the correction achieved using an approach that incorporates digital painting varies widely since it depends on the individual skills of the person making the correction. Also, such techniques require expensive workstations having dedicated, special-purpose hardware. While these techniques work for their intended purpose, they are relatively expensive and impractical for persons unskilled in digital painting.
In view of the foregoing, a need exists for a process to remove blotchy spots from a digital image without requiring technical skill, artistic ability, or manual dexterity on the part of the user. Such a system would allow an unskilled user to remove such spots from a digital image while maintaining overall image quality without requiring undue expense or the assistance of a specialist.